|
Grade beam soil pressures
|
|
Source：Internet Author：Unknow Pubdate：2010-07-10
|
|
PSU08 (Structural)
11 Jun 10 8:02
I am design multiple grade beams running between spreadfooters. The spreadfooters are for building columns which are isolated from the mat foundation that I am also design. So the spreadfooters and grade beams are the perimeter of the mat.
The spreadfooters go below the frost depth (2m) and the grade beams are just in place for the mat foundation, obvioulsy I can't make the mat 2m thick for the frost depth.
My question is should I apply the at-rest pressure to both sides of the grade beam, or apply at-rest pressure on one side and active or passive on the other side??
Thank you
dcarr82775 (Structural)
11 Jun 10 15:44
Can you thicken the mat around the edge? If not, is that the only portion of the mat that will be on fill?
字串4
Ron (Structural)
11 Jun 10 18:26
First...they are "footings" not "footers". Next, why are you concerned about lateral earth pressure when the structural member is only 1 or 2 meters below grade? The lateral pressure is negligible compared to the other loads.
PEinc (Geotechnical)
11 Jun 10 20:28
Also, doesn't a grade beam usually have about equal backfill on each side? Therefore, little or no lateral earth pressure? www.PeirceEngineering.com
paddingtongreen (Structural)
11 Jun 10 23:30
There is the construction condition when they often backfill and tamp the inside so they can go ahead with the slab, and follow up on the outside later. Michael.
Timing has a lot to do with the outcome of a rain dance.
PSU08 (Structural)
14 Jun 10 6:59
字串2
dcarr82775: We are not able to thicken the mat around the edges due to the clients instructions. Also, the entire mat will be on fill.
PEinc & paddingtongreen: There shall be equal back fill on each side of the grade beam but as paddingtongreen has said the inside will be backfilled first during the construction process.
Ron: I have heard the use of both "footings" and "footers". Also, the lateral earth pressure is not negligible compared to other loads. The only other loads acting on the grade beams is the surcharge load. I might not have explained the grade beam properly.
I have a 2m heigh pier with a 0.6m spreadfooting. Say, the top of the pier is @ EL. 300mm, than grade is @ EL. 0mm. The grade beam will extend from grade to the bottom of the spreadfooting. So, it is not between just the spreadfooting itself but between the piers also.
字串3
BigH (Geotechnical)
14 Jun 10 7:56
I'm sorry - have just never heard anyone worry about this type of problem for the shallow depths involved. Interesting (?) I have prepared quite a few geotechnical reports and have read so many more by a variety of experienced geotechnical engineers and, having a pretty extensive library, have never seen this condition addressed.
PSU08 (Structural)
14 Jun 10 11:34
They might be shallow depths but the spans, I consider to be large. The load on the grade beam is 1.5kip/ft...the grade beam length is 30'.
I've attached a sketch to clarify what I'm doing.
- http://files.engineering.com/getfile.aspx?folder=247199d6-a47b-4a51-8430-82
PEinc (Geotechnical)
14 Jun 10 23:17
字串7
6' of unbalanced fill? Sorry, but that's a retaining wall/foundation wall, not a gradebeam. www.PeirceEngineering.com
PEinc (Geotechnical)
14 Jun 10 23:19
Sorry, make that 6'-9" of unbalanced fill. Even worse. www.PeirceEngineering.com
apsix (Structural)
15 Jun 10 2:31
My question is should I apply the at-rest pressure to both sides of the grade beam, or apply at-rest pressure on one side and active or passive on the other side??
My answer; apply active pressure on the inside only.
Why would you use lateral loads on both sides when designing for the temporary case with backfill on one side only?
PSU08 (Structural)
17 Jun 10 6:40
Well our client for this project refers to the "retaining wall" as a grade beam....therefore as do we. Also, when soil surrounds the object (inside & outside), I don't consider that a retaining wall. But I agree when the inside is filled with backfill and the outside is not...that is considered a retaining wall 字串6
Thank you apsix for answering my question.
I was applying lateral loads on both sides (at-rest pressure on the outside and active pressure on the inside) for a worst-case design.
PEinc (Geotechnical)
17 Jun 10 12:07
The worst case would be at-rest pressure on one side and no pressure on the opposite side. I would check it for active pressure on one side and air on the opposite side. I would also make sure that the backfil on the inside of the "grade beam" gets compacted with a smaller, walk-behind tamper. However, I would recommend that both sides of the grade beam be backfilled concurrently and somewhat evenly. www.PeirceEngineering.com
TDAA (Geotechnical)
17 Jun 10 16:23
PEinc has the right approach. At rest on one side would be the case I would design for. If you use loads on opposite sides, the forces cancel, and would definitely not be worst case. 字串5
Only use active if you expect the grade beam to deflect due to the loading. At rest will result in more force, and designing for it should decrease the potential for wall deflection.
And as Ron said (first thing I thought when I read the OP) is that they are footings. Footers are at the bottom of the page. Just because people use the wrong term does not make it right.
PSU08 (Structural)
18 Jun 10 7:13
Thanks everyone for the comments back, I appreciate it.
（Click：）
|
| Previous：Length of grade beams on piers Next：HSS member ordinary moment frame (OMF) |
|
[ Add TO Favorites]
[TOP] [PRINT]
[CLOCE WINDOWS] |
|
|
|
